Genetic screening is an important tool to identify new genes or mutant alleles of known genes that underlie biological processes. Increasingly stem cells, particularly embryonic stem cells, have been used in these screens. Stem cells are characterised by two important properties, which are of value in genetic screening. Firstly, they have an ability to proliferate in an undifferentiated state for prolonged periods of time. Secondly, stem cells are pluripotent. This means that they are capable of differentiating into any cell of the mesoderm, ectoderm or endoderm. As a result, pluripotent cells can develop into any cell of the body.
It is at present however, very difficult to screen for recessive mutations in mammalian cells. The main reason is that mammalian cells are diploid, meaning that each cell has two copies of each gene. As a result, the phenotypic traits of heterozygous recessive mutations are masked by the second copy of the gene.
A solution to screen for recessive mutations is to use a haploid mammalian embryonic stem cell. Haploid stem cells are stem cells that in contrast to diploid stem cells possess only one copy of each gene. As a result, the phenotypic traits of recessive mutations are essentially unmasked, and as a result the underlying genes can be easily identified and studied.
Haploid embryonic stem cells have been obtained from fish as described in Yi and Hong (2009). In this paper, the authors developed haploid embryonic stem cell lines from the medaka fish (Oryzias latipes).
However, notably, at present, no such cells have been obtained from mammals and fish haploid embryonic stem cells can not be used as a substitute. Unlike placental mammals, fish are oviparous animals and have no placentation. As a result, embryonic development differs substantially from fish to mammals. Furthermore, key features such as sex determination and dosage compensation are also regulated differently and implantation, placentation and genomic imprinting are all absent in Medaka fish.
The use of a near-haploid tumour cell line for genetic screening has been described by Carette et al., (2009). In this paper, insertional mutagenesis was used to generate null alleles in a human cell line haploid for all chromosomes except chromosome 8. Therefore, the cell line described in this paper was not a true haploid. Furthermore, as the cell line carried genetic rearrangements and a transformed phenotype, its usefulness in genetic screening in developmental system is limited.
A number of reports have also found near-haploid cells in a variety of human tumours, Nonetheless, except for one case, no near-haploid cell lines have been derived from these tumours (Sukov et al., 2010).
Finally, Kaufman et al. (1983) describe the production of pluripotent cell lines from haploid embryos from parthenogenetically activated oocytes. However, the cell lines even at early passages of the cells were diploid not haploid.
Accordingly, there exists a need to produce mammalian haploid embryonic stem cells. Such cells would enable genetic screening for recessive mutations in a developmentally relevant context. For example, it is envisaged that such cells would be an important tool in identifying new genes involved in signalling pathways, developmental decisions and cell cycle regulation. Moreover, there is a need to produce haploid stem cells without tumour-derived mutations, genomic rearrangements or oncogenes. Stem cells with such characteristics are obtained when cells are derived from tumour cells. Accordingly, there exists a need to develop mammalian embryonic stem cells with a normal karyotype.